1st off, I would like to see a topic thread “what were you thinking?” It would be a great place for inspectors to post images and stories of the crazy things we find. I would like to post this picture I took yesterday. It is a pool pump, heater, service panel all grounded to conduit, yeah, you guessed it, the SCH 40 plastic type.
It looks like it is used to splice that ground.
I was just going to say the same. Doesn’t mean it was intended to bond to the pipe.
So… what were you (OP) thinking?
This is why we don’t have a “What were you thinking?” thread. 
But its gray… Just like conduit so it’s ok.